1. Field of the Invention
The present invention relates generally to a method of producing an electrochemical cell using a solid electrolyte, more particularly, to a method of producing a porous measuring electrode which is used on such an electrochemical cell for detecting or determining the concentration of a desired component contained in a subject gas, and which is exposed to the subject gas.
2. Discussion of the Prior Art
In the art of detecting the concentration of a desired component contained in a subject gas, there is known a gas sensor incorporating an electrochemical cell which has a solid electrolyte body and a plurality of electrodes formed on the solid electrolyte body. For determining the concentration of oxygen in an exhaust emission produced by an internal combustion engine of a motor vehicle as a result of combustion of an air-fuel mixture, for example, an oxygen sensor is commonly used which employs an oxygen-ion conductive zirconia ceramic as the solid electrolyte body. This oxygen sensor is operated to sense the oxygen concentration, according to the principle of an oxygen concentration cell, or by utilizing an electrochemical oxygen pumping action, or such an oxygen pumping action combined with the principle of diffusion of the measurement gas, i.e., a restricted flow of the measurement gas due to a flow resistance provided by suitable means.
In an internal combustion engine, such an oxygen sensor as described above is generally used for the purpose of accurately regulating the air/fuel (A/F) ratio of an air-fuel mixture supplied to the internal combustion engine, so that the air/fuel ratio is maintained at an optimum value. Since the oxygen concentration of the exhaust gases emitted by the engine has a known relationship with the air/fuel ratio of the air-fuel mixture, the air/fuel ratio can be determined by detecting the oxygen concentration of the exhaust emission. The output of the oxygen sensor, namely, an electromotive force induced in the electrochemical cell is indicative of the air/fuel ratio, and is fed back to a control device for regulating the amount of supply of the fuel to the engine. To this end, the characteristics of the oxygen sensor in connection with the electromotive force relative to the oxygen concentration of the exhaust emission, and the time of a response of the sensor to a change in the oxygen concentration are basically important for accurate regulation of the air/fuel ratio of the air-fuel mixture. In particular, the oxygen sensor is required to demonstrate an excellent operating response In addition to these basic characteristics, the magnitude of polarization upon application of an electric current to the electrochemical cell is also a key factor of the sensor, from the standpoint of protecting the solid electrolyte body of the cell against deterioration due to the polarization. Further, it is recognized that the sensing accuracy and operating response of the oxygen sensor can be improved by reducing a deviation of the electromotive force induced by the electrochemical cell, from the theoretical or nominal value at a varying level of the oxygen concentration of the exhaust gases.
In the electrochemical cell used for sensors such as an oxygen sensor as described above, the electrodes provided on the solid electrolyte body are usually given a porous structure for obtaining a relatively large number of triple points (points of reaction) among the solid electrolyte, a metal of the electrodes and the measurement gas. For instance, the porous electrodes are formed by co-firing unfired layers of the electrodes together with an unfired body of the solid electrolyte material. This co-firing method to obtains a porous structure of the electrodes due to on a difference between the sintering temperatures of the material of the solid electrolyte body and the material (in the form of a paste) of the electrodes. However, since the sintering temperature of the solid electrolyte material is higher than that of an electrically conductive metal such as platinum contained in the material of the electrodes, the conductive metal of the electrodes is sintered at the co-firing temperature, whereby the fired electrodes cannot be made sufficiently porous. Accordingly, the electrochemical cell having the thus formed electrodes, and the sensor using this cell do not provide satisfactory characteristics, particularly in terms of the operating response.